The standard two-part transformer casings now in use for high voltage transformers in the communications field consist of a main casing which has a groove or channel of rectangular cross-section into which the cover is slidably inserted. During manufacture, hot liquid potting material, e.g. epoxy, is poured into the case which is then subjected to a vacuum to remove entrapped air. In the past, leakage around the casing-cover seal occurred frequently during this process. To eliminate such leakage, sealers or adhesives were necessary which added to the cost of production.